For many years tars and pitches have been used to fill the joints and cracks between adjacent concrete slabs which are used as highway and airfield pavements and to form films or waterproofing membranes on concrete slabs. Tars and pitches, in their native form, have many unsatisfactory characteristics for this use. For example, tars and pitches, in their native form, get very brittle in cold weather and tend to crack and lose the bond between the tar or pitch and the concrete slab. As a result, water penetrates the joint and freezing and thawing cycles causes the slabs to break up. In very warm weather tars and pitches become soft and tacky which, in turn, causes them to be extruded from the sides of a crack when exposed to traffic and causes tar to stick to vehicle wheels causing a problem known as tracking in which the tar or pitch is removed from the crack by adhering to vehicle wheels and to spread over the surface of pavement by contact between the wheel surface and the pavement.
Native tars and pitches are particularly vulnerable when used in airfield concrete pavements for modern airports because they are soluble in jet fuel and because they liquefy in the heat of a jet engine blast.
Many of the above-mentioned problems have been solved by the advent of blends of coal tar pitch and vinyl chloride polymers, hereinafter PVC. Blends of coal tar pitch and PVC form a rubber-like gel that remains flexible in cold weather and rubber-like in hot weather. The PVC-coal tar pitch mixture exhibits good adhesion to concrete and good cohesion so that the joint between adjacent slabs remains sealed against moisture penetration. In addition, the PVC-coal tar pitch gel withstands the heat of a jet engine blast and is not soluble in jet fuel. Generally, the coal tar pitch-PVC composition includes solid particulate fillers such as clays and these fillers both occupy volume and contribute to the texture and function of the composition. In addition, the blends include a plasticizer which is usually a non-volatile liquid that is incorporated in the structure of the final material in such a manner that it will prevent the final composition from becoming too brittle. Plasticizers for use in PVC-coal tar pitch compositions typically are octylphthalates.
It is currently thought the PVC-coal tar pitch blends obtain their desirable properties by a physical relationship between the components. PVC is normally added to coal tar pitch in the form of a particulate solid. Since coal tar pitch is normally a solid at room temperature, it is necessary to heat the coal tar pitch to make it liquid phase so that the PVC and other components of a mixture can be blended into it. The hot coal tar pitch causes the PVC to dissolve and upon cooling the PVC forms a molecular network within the coal tar pitch to produce the rubbery product that has been found desirable as a sealant.
It has been found desirable and almost essential that the blend of ingredients used in a joint sealant be made at the factory rather than in the field on the site of where the materials are to be used. Sophisticated measuring and blending techniques are essential to have the materials in the right proportion and in the right condition for being applied to a joint, for example, in an airport runway. As a result, manufacturers generally compound the entire blend and then place it in containers to be shipped to the site of application. At the site of application, the material is removed from the container and placed in a heated vessel and when it is hot enough to have good consistency for pouring, it is poured from the vessel into a joint between adjacent concrete slabs. When it cools in the joint the composition obtains its desirable rubbery, adhesive, cohesive properties and forms an effective seal for many years.
Although the solution of PVC and the formation of the network-like structure is a physical reaction, PVC-coal tar pitch mixtures also react chemically with each other to form a thermosetting, cross-linked compound which is not a good joint sealant composition but it is primarily undesirable because it causes the joint sealant composition to set up in the heating vessel. This cross-linking reaction is promoted by high temperatures. Accordingly, a PVC-coal tar pitch mixture has a limited high temperature life and it is essential that exposure of such mixtures to high temperatures be limited in the manufacturing process to give maximum "pot life" in the field where it is being applied.
It is known from U.S. Pat. No. 3,549,575 and its reissue U.S. Pat. No. Re. 29,548 that in compounding joint sealant compositions a liquid phase can be maintained at room temperature by using a particular boiling range pitch which exhibits an anomoly in that it is liquid at room temperature even though higher and lower boiling pitches are solid at room temperature. This pitch identified as D pitch in the patent is from coal tar and it boils between 355.degree. and 450.degree. C. at atmospheric pressure. The liquid nature of D pitch permits blending PVC particles into it at low enough temperature to prevent PVC from dissolving. The boiling range of D pitch also is high enough to exclude almost all naphtalene compounds from its composition which is necessary because naphthalenes cause PVC to dissolve and would result in gelling of the composition in its shipping container.
Although D pitch is liquid at or slightly above room temperature, it is a viscous liquid and it is desirably made more fluid by blending it with a compatible, less viscous liquid. As stated above, this has been done in the past by blending D pitch with an octylphthalate, for example, dibutyl phthalate. The phthalates not only make the pitch more fluid but have the desirable after-effect of being a plasticizer for the final product. Unfortunately, octylphthalates are very expensive and constitute a significant portion of the total cost of the joint sealant.
Having a very fluid pitch before it gels is very important not only from the point of view of being easy to use but also from the point of view of the character of the final product. Pitch that is liquid in the container is very readily poured into a heating vessel at the site of application, it has a long "pot life" because no cross-linking reactions have yet been effected, and if it is very fluid liquid it readily fills joints between adjacent concrete slabs without causing bubbles to be trapped within the sealant composition so that the final seal between the joints of concrete slabs is complete and without bubbles or blisters. The composition of this invention not only avoids the formation of bubbles and blisters when it is applied to a joint, but it additionally has been found to resist blister formation upon long exposure to heat and ultraviolet radiation.